The present invention relates to a data transfer system for controlling data transfer from one memory to another. More particularly, the present invention relates to a data transfer control system that extracts a specific field rapidly out of transferred word data.
For the transfer of data from one memory (hereinafter, referred to as a source memory) which the data to be transferred are stored on to another memory (hereinafter, referred to as a destination memory), a processor accesses to the source memory. The source memory stores the data by each page having a predetermined structure as a unit. The processor directs a page address register which page in the source memory should be accessed. A synchronous data transfer controller comprises an address generation circuit. A time-source-destination table (hereinafter, abbreviated as TSDT) has a time designation field, a source designation field, and a destination designation field. The address generation circuit designates an address in the source memory according to the source designation field and page designation information stored in the page address register. The data taken out of the source memory are transferred as transferred data to an output port designated by the destination designation field. The transferred data are sent to a pre-fetching register. Word data in a page on the source memory are consecutively transferred according to the fields defined in TSDT.
Conventional data transfer control systems extract a desired portion of the data by masking an unnecessary portion of the same data after the transfer of the data from the source memory to the destination memory. More specifically, TSDT of the conventional data transfer control systems is directed to designate only an address in the source memory and a storage region in the destination memory in which the transferred data are stored. This means that the whole word data at a designated address in the source memory is transferred to the destination memory, which makes it necessary to carry out shift operation or mask operation of the transferred data after completion of transfer in order to extract a desired field out of the transferred data.
The above-mentioned conventional technique designates the address where the data to be transferred are stored in the source memory, but not the portion of the data to be transferred. Thus the whole word data are transferred to the destination memory. For this reason, the destination memory should have the same bit width as the source memory. The designation memory of such structure cannot be reduced in size.